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Analysis of Cross-Shore Movement of Natural Longshore Bars and Material Placed to Create Longshore Bars

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This study develops empirical predictive expressions for design of nearshore berms formed of dredged material. Such berms are placed in the form of long linear sand bars and are expected to behave as natural bars. The study examines the cross-shore movement of natural longshore bars at Duck, North Carolina. Beach profile survey data are available at approximately 2-week intervals from 1981 to 1989, together with measurements of the wave conditions. Two bars are typically present at Duck, an outer bar at approximately the 4- to 5- m depth and an inner bar at 1- to 2-m depth. These bars tend to move offshore during storms and onshore during periods of lower waves. A method is introduced to define bar-type features unambiguously for analysis of field data. The method uses an equilibrium profile shape defined in terms of decreasing grain size with distance offshore. Analysis is made of bar movement, and criteria previously developed by the authors to predict beach erosion and accretion are found applicable if the value of a multiplicative empirical coefficient in each criterion is modified. The results show that onshore movement of bars is more probable than previously estimated, indicating a wider possible range of wave conditions favorable for beach nourishment through creation of nearshore berms. The predictive criteria developed from the east coast beach successfully described the observed onshore movement of a berm placed at Silver Strand Park, California. The criteria, expressed as nondimensional parameters, appear to have applicability to sites where bars and constructed submerged berms are modified primarily by wave action.

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  • Physical and Dynamic Oceanography
  • Civil Engineering

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